The human spine is a somewhat flexible structure including thirty-three vertebrae. The vertebrae are separated and cushioned from each other by fibro-cartilaginous structures referred to as intervertebral discs.
Artificial intervertebral discs, or intervertebral "spacers", are implanted in a disc space after the removal of a diseased or damaged natural intervertebral disc. Various types of artificial discs, or spacers, are described in U.S. Pat. No. 3,867,728, issued Feb. 25, 1975 to James A. Stubstad et al.; U.S. Pat. No. 3,875,595, issued Apr. 8, 1975 to Edward C. Froning; U.S. Pat. No. 4,309,777, issued Jan. 12, 1982 to Arun A. Patil; U.S. Pat. No. 4,349,921, issued Sep. 21, 1982 to J. David Kuntz; U.S. Pat. No. 4,743,256, issued May 10, 1988 to John W. Brantigan; U.S. Pat. No. 5,071,437, issued Dec. 10, 1991 to Arthur D. Steffee; U.S. Pat. No. 5,123,926, issued Jun. 23, 1992 to Madhavan Pisharodi; and U.S. Pat. No. 5,192,327, issued Mar. 9, 1993 to John W. Brantigan.
In another approach, the damaged disc is left in place and two capsules are implanted side by side, and spaced apart, in the damaged disc. The capsules are provided with (i) an outer layer of inert fibers intermingled with a bioresorbable material which attracts tissue in-growth, (ii) a bladder enveloped by the outer layer, and (iii) a thixotropic gel filling the bladder. See U.S. Pat. No. 4,772,287, issued Sep. 20, 1988 to Charles D. Ray.
In still another approach, similar in some respects to that described immediately above, fusion cages are placed side by side in a damaged disc between two normal natural vertebrae. These fusion cages comprise threaded members having holes therein. The natural vertebrae, in time, fuse with the cages. See U.S. Pat. No. 5,015,247, issued May 14, 1991 to Gary K. Michelson; U.S. Pat. No. 5,026,373, issued Jun. 25, 1991 to Charles D. Ray et al.; U.S. Pat. No. 5,489,308, issued Feb. 6, 1996 to Stephen D. Kuslich et al.; and U.S. Pat. No. 5,055,104, issued Oct. 8, 1991 to Charles D. Ray.
In U.S. Pat. No. 5,059,193, issued Oct. 22, 1991 to Stephen D. Kuslich, there is disclosed an expandable spinal implant for insertion into a bore formed between opposing vertebrae. This implant comprises a plurality of ribs deformable, in response to a deforming force, from a first shape to a second shape, with the ribs in the first shape being generally linear and the ribs in the second shape being arched outwardly in a ball-like configuration.
Despite such developments in the art, there remains a need for an effective intervertebral spacer and, in particular, a spacer which may be introduced into the patient's body through a relatively small incision.